Limitation by nutrients

Often, the rates of fertilization in intensively managed agriculture are intended to satiate the needs of crop plants for these chemicals, so their productivity will not be limited by nutrient availability. However, excessive rates of fertilization have important environmental costs. These include the contamination of ground water with nitrate eutrophication of surface waters caused by nutrient inputs (especially phosphate) acidification of soil because of the nitrification of ammonium to nitrate large emissions of nitrous oxide and other nitrogen gases to the atmosphere, with implications for acid rain and Earth s greenhouse effect and the need to use herbicides to control the weeds that flourish under artificially nutrient-rich conditions. 

For a given medium a cell line can be characterized by a maximum specific growth rate, which is the observed growth rate in the absence of any limitations by nutrients or any inhibition by metabolites. This maximal growth rate is related to the doubling time (t ) of the cell by the relationship ... 

The duration of the spring bloom is limited by nutrient availability and/or grazing by zooplankton. Phytoplankton growth and abundance then decline to lower levels, which are maintained throughout the summer by nutrient recycling within the euphotic zone. In some locations, limited mixing in autumn can stimulate another small bloom, before deep winter mixing returns the system to its winter condition. 

In tropical waters, vertical stratification persists throughout the year and production is permanently limited by nutrient supply rates, which are controlled by internal recycling and slow upward diffusion from deep water. Under these conditions, productivity is low throughout the year. 

The quota can vary between a minimum content kQ and a maximum <2max, and p is 1 for a limiting nutrient and greater than 1 otherwise. The ratio <2max/fcg determines a population s storage capacity for a given nutrient. The ratio k-Qp/ko helps to determine relative limitation by nutrients j1 and j2. For example, Sji/Sj2other factors also play a part. 

Mixing within particles is impossible and growth can be limited by nutrient diffusion Ability to remove metabolic heat is restricted, leading to overheating problems Process control can be difficult due to difficulties in making on-line measurements, and in measuring biomass... 

We can see that for type 1 processes, high growth rate is obligately linked to a high rate of product formation. Indeed, this is the case for all products produced by a fermentative mode of metabolism, eg ethanol, lactic add, acetone. Chemostat studies have shown that for most aerobic processes when growth is limited by some nutrient other than the carbon source, the yield of product decreases with increase in spedfic growth rate (p or D p = dilution rate (D) in chemostat culture). Conversely, both the biomass yield and the spedfic rate of substrate utilisation (qs g substrate g biomass-1 h-1) increase with spedfic growth rate. 

The possible effects of increased atmospheric CO2 on photosynthesis are reviewed by Goud-riaan and Ajtay (1979) and Rosenberg (1981). Increasing CO2 in a controlled environment (i.e., greenhouse) increases the assimilation rate of some plants, however, the anthropogenic fertilization of the atmosphere with CO2 is probably unable to induce much of this effect since most plants in natural ecosystems are growth limited by other environmental factors, notably light, temperature, water, and nutrients. 

As cooling occurs in the late fall and early winter, the thermal stratification breaks down, permitting mixing of the deep and surface layers. This allows the surface layers to be replenished with P. During the winter months, biological productivity in a temperate lake is limited by the availability of light rather than nutrients. 

Kluiyama H. Slaughter J.C. (1995) Control of cell morphology of die yeast Saccharomyces cerevisiae by nutrient limitation in continuous culture. LettApplMicrobiol, 20, 37-40. 

Saponins and phenolics also have anti-microbial properties (Chung et ah, 1998) and have been associated with reduced rumen functions (Klita et al., 1996 Reed, 1995), thereby limiting the nutrient quality of forages. Also, carbohydrate complexes with hgnins and other compounds reduce carbohydrate utilization by rumen bacteria (Cornu et al., 1994). The influence of such anti-microbials on rumen functions can affect small intestine characteristics by altering nutrient concentrations (Barry and McNabb, 1999). 

In natural ecosystems, microbial growth and metabolism may be limited by the concentrations of inorganic nutrients such as nitrogen, phosphorus, or even iron. Systematic investigation of these... 

M. van Noordwijk and S. C. van de Geijn, Root, shoot and soil parameters required for proeess-oriented models of crop growth limited by water or nutrients. Plant Soil I8S (1996). 

Poly(3HB) is usually produced in a batch or fed-batch regime. These types of process control are derived from the general observation that overproduction of poly(3HB) occurs when cell multiplication is limited by an essential nutrient and the carbon substrate is available in excess. Batchwise production has an advantage in that a high poly(3HB) content can be reached. One disadvantage is the quality of the product, which can vary from one batch to the next. This can be overcome by a continuous process. Continuous production is basically possible if ... 

Finally, a novel process has been described for efficient photoconversion of low-grade organic materials such as waste biomass into polyesters. In this process, dry biomass has been thermally gasified which resulted in gas mixtures composed primarily of CO and H2. Photosynthetic bacteria photoassimilated components of the synthesized gas into new cell mass. Under unbalanced culture conditions, when growth was limited by several nutrients, up to 28 % of the new biomass was found to be poly(3HB) [37]. 

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